Various types of separation media are available for use in isolating target substances in chemical or biological samples. In molecular biology, various materials are used in fractionation procedures, including agarose and polyacrylamide in electrophoretic fractionation, and gel permeation, ion exchange and affinity materials for chromatography. Among such materials, both organic polymer and silicon-based particles find use in separations of targets such as nucleic acids. In each type of separation there is a need at some point physically to remove a liquid phase from the particles in the solid phase. This may be achieved by immobilising the particles, for example in the form of a column or on a gel plate, and eluting the liquid phase or by aggregating the particles by applying a force under centrifugation or using magnetism. The use of magnetism to separate particles from other sample components requires the particles to respond to a magnetic field. Because organic or silicon-based materials are not themselves magnetically responsive, there is a need to devise methods of production of magnetic particles which incorporate both a magnetic component and a polymer component.
U.S. Pat. No. 5,945,525 describes a method for isolating nucleic acid using silica-coated magnetic particles. Coated particles are formed by deposition of silica from a tetraethoxy silane/alcohol solution onto the surface of triion tetroxide particles of diameter 200 to 400 nm. Magnetic particles comprising a magnetic core surrounded by silica are formed by a reaction between the coated particles and sodium silicate. U.S. Pat. No. 5,352,481 describes a process for forming particles having a uniform size distribution by the so-called sol-gel method. Titania powdered core particles are coated with a silica gel by allowing an alcoholic solution of silicon tetraethoxide to be hydrolysed so that a silica sol-gel is formed as a coating.
A problem with these methods when used to produce polymer magnetic particles is that they are multi-step methods in which the amount of magnetic component and its distribution in the particles is very difficult to control. Generally, such particles have a non-uniform distribution of magnetic component because the magnetic component is situated only at the core of the particle. This affects adversely performance of the particles in separation procedures.